ANIMAL MINDS

Harvard evolutionary psychologist, Marc D. Hauser, argues that to understand what animals think and what they feel, we must ask about the kinds of selection pressures which shaped their minds and see the creature for what it is, no more, no less. Using the tools of evolutionary biology, linguistics, neuroscience, and cognitive science, he asks questions such as Why can't animals be taught to speak? How do animals find their way home in the dark? Do animals lie or feel guilty? Do they enjoy sex? Why were emotions designed into animal systems? Why are certain emotions universal and others highly specialized?

Hauser works on both captive and wild monkeys and apes as well as collaborative work on human infants. His research focuses on problems of acoustic perception, the generation of beliefs, the neurobiology of acoustic and visual signal processing, and the evolution of communication.

Along with Irv Devore, he teaches the Evolution of Human Behavior class, a Core Course at Harvard with 500 undergraduate students. The interdisciplinary course, "Science B29" (nickname: "The Sex Course"), has been running for 30 years, was started by Devore and Robert Trivers, and is the second most popular course on campus, behind "Econ 10". Section teachers over the years comprise a who's who of leading thinkers and include people such as John Tooby and Leda Cosmides, and Sarah B. Hrdy. In 1997-98, he sponsored a trial run of "Edge University" in which the students in Science B29 received Edge mailing as part of required reading in the course.

MARC HAUSER: The big questions that are on my mind are of puzzles that we have no answers for. Those questions are things like, why are humans the only species that cries with tears? It's true that the emotions supporting crying with tears are common with humans and animals, and yet we're the only species that ends up generating a physical output of the emotion. If you look at crying with tears from an evolutionary perspective, which people really have not done, you begin to get some of the answers. Unlike all of the other emotional expressions, tearing as an emotional expression is the only one that leaves a long-term physical trace; it blurs one's vision, therefore it's costly; it's very difficult to fake; and what this then converges on is an idea that the evolutionary biologist Zahavi proposed many years ago which is that signals that are costly to produce are often honest signals, because those that do not have the resources to produce them would be unable to generate them. You can look at a signal and infer its honesty based on the cost of expression. Crying with tears is potentially one of those — even for actors, it's necessary for them to have the feeling before they can generate the expression, and even then it's quite hard to do it naturally.

We know that animals have things like sadness; whether they have joy is hard to say, but they certainly have the emotions that would accompany crying with tears even if they don't have that connection. It's not that they don't tear from the eyes, because they do if the eye is physically irritated; it's that somehow in the brain there's a connection missing. To say that they lack the connection in the brain is an answer at only one level of the analysis, which is the mechanism — what brain mechanisms support tearing. The more interesting question is to take the evolutionary approach and ask why we cry with tears and other animals do not? And the answer comes from thinking that it's an expression which really conveys honesty.

For the past few years I have been using the theoretical tools from evolutionary biology to ask questions about the design of animal minds. I'm particularly interested in taking the approach that some people have had within evolutionary psychology, and saying look, this whole notion of the environment for evolutionary adaptedness which people have pegged as being associated with the hunter-gatherer period in the Pleistocene, may be true for some aspects of the human mind, but are probably wrong as a date for many other aspects. If we think about how organisms navigate through space, recognize what is an object, enumerate objects in their environment — those are aspects that are probably shared across a wide variety of animals. Rather than saying that the human mind evolved and was shaped during the Pleistocene, it's more appropriate to ask what things happened in the Pleistocene that would have created a particular signature to the human mind that doesn't exist in other animals.

I've been looking at different domains of knowledge, and asking the question, what pressures would have shaped ways of thinking in different organisms, trying to get away from the common approach to thinking about humans, human evolution, and animal cognition, which is to say, humans are unique, and that's the end of the story.All animals are unique, and the really interesting question is how their minds have been shaped by the particular social and ecological problems that the environment throws at them. For example, instead of stating that humans are unique, we ask the question: what pressures did humans confront, that no other animal confronted, that created selection for the evolution of language? Why can other organisms make do with the kind of communication system they have? Why did we evolve color vision? Why did other organisms not evolve color vision? Why do certain animals have the capacity to navigate in space with a simple mechanism like dead reckoning, and other animals need other kinds of machinery in order to get by in space?

This leads to a scientific approach to the study of animals and humans that brings the two groups of organisms together for the first time. What we're now approaching is a new period in the study of animal minds, where we can use techniques that in part have been developed within the study of humans, and apply them directly to the study of non-human animals. Conversely, methods developed on animals are being used by cognitive scientists studying humans.

Here is one example: Researchers studying cognitive development, such as Susan Carey, Elizabeth Spelke, and Renee Baillargeon, have implemented a novel technique for asking human infants, who of course lack a functional linguistic system, about how they think about the world. The technique is quite simple, and is really just a bit of magic. The idea is that when we watch magic shows such as those carried out by the great Houdini or David Copperfield, we're engaged because the magician is violating, in front of our very own eyes, all sorts of assumptions we make about the physical world. Bodies are not capable of being cut in half and then put back together again in any kind of symmetrical way, and yet that engages our attention because we expect bodies to cohere. If the logic of a magic show, or the special effects of a program on television or a movie, grabs our attention precisely because it violates our expectations, then we can ask the question, what do infants or non-human animals bring into the world in terms of their expectations about how things should work? If they too carry specific expectations, then we should be able to create a magic show and grab their attention. Importantly, they should be more interested in a magic show than in a comparable demonstration that is consistent with the way the world works.

Imagine an open stage, a screen comes up that blocks the stage, and now what happens is one object goes behind the stage, followed by a second object. Let's call them Mickey Mouse 1 and Mickey Mouse 2. In our minds we are representing two Mickey Mouse objects. When the screen is removed, we expect to see two Mickey Mouse objects. If we see three, or we see one, that would be a violation because nothing should have been added to what was going on behind the stage. It turns out that at the age of about four to five months, human infants look longer when they see an outcome of three objects, or one object, as opposed to the two objects that they appear to expect. This is at a very young age. We have run the same exact experiment with two different non-human primate species, rhesus monkeys living wild on the island of Cayo Santiago, Puerto Rico, as well as a primate called the cotton-top tamarin living in my lab at Harvard. We have found exactly the same results as the psychologist Karen Wynn found with human infants. These results raise the important question of whether the representation of number is innate. This question is important for our understanding of the mechanisms underlying developmental and evolutionary change, and also, for our understanding of the relationship between language and thought. In fact, because animals lack language, studies of their mental representations provides a beautifully clear method for exploring under what conditions language is necessary for thought and under what conditions it contributes and enriches our thoughts.

JB: Gregory Bateson said that humans can't hold more than 7 of anything in their minds.

HAUSER: What seems to be occurring with studies of human infants and animals that involve spontaneous representation of number is more on the order of four. Four seems to be the limit of what we can actually enumerate spontaneously without a mechanism that encodes each object with a particular symbol, like an Arabic numeral. Very much in parallel with some of the findings that people like Stanislas Dehaene have produced recently, we are now finding that there are some core principals that non-human animals and infants bring to the table of numerosity, which then become elaborated through both language and culture, through education and through our capacity for language. What becomes interesting is to do comparative studies on human infants and non-human animals to see at what point they diverge in their capacities. We know of course at some point humans are able to do calculus, become bankers, do their taxes, and non-human animals are not able to do that. That's not interesting. What is interesting is what happens during the course of development that separates a human child from a non-human animal. By identifying the divergence point we are able to show what cognitive ability developed in the child which failed to evolve in non-human animals. The elegance of this work is that we're now beginning to pinpoint processes that emerge during development in the human child that either have evolved in other animals or have failed to evolve. By identifying both the similarities and differences we begin to pick up a pattern of evolution which is truly unique in terms of our understanding of both our own species and the unique attributes of other species.

JB: Let's talk about what you do in your job.

HAUSER: My day job involves a couple of different strands. One of the most novel aspects of my work is that unlike researchers who restrict themselves to field studies, people like Jane Goodall, or Robert Seyfarth, Dorothy Cheney, where most of the work is done in the wild, with animals living in their natural conditions, occasionally maybe an experiment in captivity, my own work combines at least four different kinds of approaches to finding out what animals know, think, and represent.

The first is field studies. I go into the wild — places like Puerto Rico where there's a wild population of rhesus monkeys, or Africa and look at chimpanzees in Uganda or vervet monkeys in Kenya, and I use my studies in the wild to understand what kinds of problems shaped the design features of the brains of those animals in their natural habitat. Watching what they do will tell us what particular problems their brains need to solve. The same logic applies to humans of course, and is one reason why the study of the human mind must not be restricted to studies in the laboratory. We need to figure out what kinds of problems humans confronted in their past and that are recurring problems to understand how our own minds were sculpted by the forces that we have confronted. We take that same approach and we apply it to animals as well. In the wild, vervet monkeys in East Africa confront a wide array of predator, as the biologists Peter Marler, Thomas Struhsaker, Dorothy Cheney and Robert Seyfarth have documented. In response to these predators, which have very different hunting styles, they have evolved a unique alarm call system, where different kinds of calls appear to refer to different kinds of predators. We would never have uncovered that particular phenomenon if we had studied these animals in captivity where there are no predators. So I go to the field, I watch what animals do naturally, and then I come back to the lab where we can gain more experimental control and ask specific questions about their cognitive ability.

In the lab, for example, we would take the fact that animals seem to discriminate all sort of objects in their world, and ask them what features are relevant to that kind of discrimination. To give one example, we now have 30 years of studies showing that animals use tools. They use objects to extract food from their environment. But what none of these studies have ever shown is the kinds of representations that the animals bring to the task of tool use. Now here's the question: for humans we know that when we have an artifact like a tool there are certain features of the object that are relevant to the tool, and certain features that are irrelevant. A simple example is that most dishwashers are white, but if we came into our home and found a dishwasher that was rainbow colored we wouldn't say "That's no good. Can't do my dirty dishes in that thing". We know that color is an irrelevant feature of what makes a good dishwasher as opposed to a bad dishwasher. The question then becomes, when we see animals, for example chimpanzees in the wild using stones to crack open nuts, if we presented them with a stone as opposed to a sledge hammer, would they see that the sledge hammer has better design features for the task than does the stone? Would they prefer the sledge hammer? Would they realize that if we painted the stone red it wouldn't make any difference at all to its functionality? So what we've done is we've taken that problem, gone into the lab, and systematically manipulated all features of the objects, both relevant and irrelevant, to see whether animals make a decision based on those features. When we run those experiments we see that animals are, in fact, quite sensitive to the functionally relative features, ignoring differences that have no impact on the task. We can take a problem from the wild and bring it into the lab, and systematically assess how the representation is actually operating in the animal's mind.

A third step in this sort of program of research is to take these problems to a more neuro-physiological level. In collaboration with neuro-scientists throughout the United States and internationally, we've begun some experiments to look at how the brains of rhesus monkeys in particular decode information about their vocalizations. Using recordings from neurons in the various auditory areas of the brain we play back vocalizations from their repertoire and see how their nervous system actually decodes that information. This is relatively new work; for a long time now we've gained an incredible amount of knowledge about the neurobiology of vision from using rhesus monkeys as a model. Almost nothing has been done in terms of auditory function. And yet when we think about language we really know nothing about the evolution of language and speech because we know very little about the neuro biology that underlies this fantastically complex system. Now, for the first time, we have the tools to acoustically probe how the non-human primate brains actually encode and decode these vocalizations. There is a long history of this kind of work with insects, birds, frogs, and bats, but almost nothing on our closest living relatives, the primates.

The fourth is comparative studies that I alluded to earlier where we do the same experiments with human infants that we can do now with non-human animals. This allows us to make direct comparisons of the abilities of humans and non-human animals with the same kind of task. As an example we explore the numerical abilities of human infants and non-human animals, using these kinds of techniques of magical violations to explore the kinds of representations they bring to the task of enumeration.

Thus, we have a 4-pronged approach to understanding the design of animal brains that goes from the field taking what ecological and social problems would have designed their brains the way they are, back into the lab to more systematically evaluate what we think are the representations underlying those problems in the wild, to the neuro-physiological level, which gets at a very mechanistic level of how a brain may generate mental states, and then finally comparing directly non-human animals with human infants so we can compare developmental processes with evolutionary processes.

JB: Do animals think and why do we care?

HAUSER: There's always this question about whether animals think and if so what are they thinking about. One of the things that's held back studies of animal cognition in terms of their integration with the cognitive sciences is that people have been obsessed with the question of intelligence, in the same way that we've been obsessed with human intelligence. That's often been a detour in our understanding of cognition. The same has been true for animal studies. People have been concerned with questions like are dogs smarter than cats; are dolphins smarter than pigeons; are chimpanzees smarter than dolphins; are we smarter than them; if so when did we become smarter than them? These are not good questions. The more interesting question is to evaluate what animals have to deal with in order to survive. Each species is intelligent in its own way. The question for me is not "Are animals intelligent, and do animals think?", but rather more specific questions we can answer, like "Can animals remember things? And if so how far back in time can they remember? Do they have memories of what they were like when they were young? Can animals learn about abstract properties of the world, and if so, why would they learn about them?" These are questions that we can ask, and I believe, answer with the tools of science. If you then want to say that, given those abilities, those are intelligent animals, that's fine. If you want to say that here are the ways in which animals communicate and that looks like language, that's fine with me too. It seems to me, however, that a more interesting question is to say, when humans communicate they have this interesting capacity to refer to things in their world. I can talk about a chair, I can talk about my past, I can talk about the future in a very abstract way. Do animals have that capacity? If they do then it looks like our system of communication. Those seem to me much more interesting problems, and tractable from an empirical perspective. We can say "Do animals have the moral emotions? Can they empathize? Do they feel guilt? Do they feel shame? Are they loyal? Do animals have the capacity to cooperate? Do they engage in reciprocal altruism? These are questions we can answer. They are difficult questions, but we can at least try to make some headway, and in many cases, we have made a great deal of progress. So I don't ask the question do they thin?; I don't ask the question are they intelligent?. I ask more specific questions that have to do with particular cognitive mechanisms that we know we can identify in humans, both infants and adults, and therefore we've got at least sensible questions to ask and therefore address. That's how I approach that question. I don't want to set up a taxonomy of intelligence among animals; I want to compare them in terms of their abilities. If somebody else wants to impose a taxonomy of intelligence that's fine.

Now why should we care? There's a large group of people out there who love their pets and think that their dogs are Einsteins, and I want to show them that they should not be satisfied with their own intuitions. Our own intuitions are often not good guides to what animals are doing, in the same way that our own intuitions are often poor guides in terms of how human infants think about the world. One of my goals is to reach the public in a way that will make the science more palatable and make it less controversial. The way to do it is the following: for people who study animals in the wild, or study animals even in the lab, we are often flooded by incredible observations of what they do or they don't do, and it's very tempting to interpret those in specific ways. The lay public has the tendency to think that scientists diss those one-off observations. If they come to you and tell you, look, my dogs just did the most fantastic thing. I left them six hours from our house and they found their way home. Isn't that amazing? Scientists will say, well, no, because it's only one observation and we can't really do much with one observation. That's a mistake. It's not that scientists think that one observation is irrelevant; it's that it's unsatisfying. What I want to convince people of who are interested in animals is that they should be unsatisfied too. And what I do is give an example of a personal experience that I had with an animal that just whetted my appetite for more questions, and I want the lay public to be equally whetted by these observations.

The example is the following: I was an undergraduate, and working at a tourist spot called Monkey Jungle, in Florida. I had to make some money because I was quite poor, so I decided to take a job raking whatever dropped below the cages, away from the cages, as well as being the feeder. One day I noticed that a spider monkey, a species that inhabits the rainforests of South America, was intently looking at my raking. I didn't think that she was that interested in the raking, so I thought maybe she was interested in me. She had a mate, who wasn't paying much attention. I put the rake down and approached the cage. As I approached, she approached and sat on the other side of fence from me. She looked me in the eyes, and put both of her arms through the cage and wrapped her arms around my neck, and cooed. She sat there for quite a long time, a few minutes. Her mate then approached; she unwrapped her arms, smacked him in the head, and went back to putting her arms around my neck. You can imagine the kinds of thoughts that might go through your head in this experience; you're really connected with the animal, they're in love with you, there's a million possibilities.

JB: Maybe she wanted something from you.

HAUSER: Maybe she wanted more food, maybe the previous trainer had trained her to do this. Maybe she was trying to make her mate jealous, you know, new boy on the block, — there are all sorts of possibilities. It is interesting to try to narrow some of the possibilities. Simple experiments. If somebody else came by and was raking the enclosure would she do the same thing? What if it was a female? What if it was a young boy? What if it was an older man? These are the kinds of things you could quickly eliminate as possibilities. If it's specifically me, why me? Is it something about the way I behave; is it something about the way I look? Is it something about the way I smell? Let's change my clothes. Is it just me with certain clothes on; I was in the same clothes every day. Very quickly one could eliminate a lot of uninteresting possibilities, and begin to narrow it down to some interesting possibilities.

Philosophers often like to use examples of animals to show how difficult it is to understand the representations and thoughts of creatures that lack language. Moreover, some philosophers will claim that in the absence of language, there can be no thought. If that's true we're in a very difficult bind when it comes to understanding animal thought. In fact, some would claim that the entire enterprise is bankrupt. Yet there is a long history of research on humans where tasks have been developed to figure out what humans are thinking in the absence of language, and a massive amount of work on human infants, who have yet to express their linguistic capacity. What I argue is that some of the most profound problems having to do with the human mind can only be addressed by studying animals, not humans. There are three threads to this claim. Thread number one: there are many cases where people working on humans want to claim that a particular kind or style of thought depends upon language. My argument would be that if you want to make that claim the only species you can test that hypothesis on is an animal — not humans. Not human infants, who although they have yet to develop competence with language, have nonetheless evolved a brain that is suitable for language, and therefore it's not quite the appropriate test. Similarly, studies of brain-damaged patients, who don't have production or comprehension of language — they have a language aphasia — are also not good subjects, because they have a brain that developed with language. Therefore if you're interested in the connection between language and thought, you must test that hypothesis on species that lack language. So in our lab and in the field, along with many other scientists like Dorothy Cheney and Robert Seyfarth, as well as others, we have explored particular kinds of thought that appear to require language in organisms like non-human primates, or non-primate animals, to see whether they have those capacities, and increasingly, there are now elegant demonstrations of such representational capacities and thoughts, but without language.

Thread number two. The second reason why people studying humans must focus on studies of animals is because there are an awful lot of claims about the special nature of a particular human thought process. Debates beginning in the 1960s and 70s, and continuing into the present, focused on the special mechanisms underlying speech. People claimed that our ability to make categorical distinctions between phonemes, like ba and pa, is due to a special mechanism underlying speech. The first hit on that claim was due to an experiment that was run by Pat Kuhl. Kuhl ran experiments on chinchillas and macaques, showing that they have the same exact perceptual abilities as do humans, given the same set of stimuli. This initiated a program of research aimed at identifying whether we can make the claim that a particular mechanism is special to humans or not. The only way to really address this claim is by studying animals.

Thread number three, which is much more familiar to psychologists and neuroscientists, is the idea that certain kinds of experiments are either unethical or logistically too difficult to run on humans, but can be conducted with an animal. Although the ethical issue usually dominates this debate, it is equally important to consider how studies of animal might complement those on humans, because we may be able to do better experiments on animals, due to the level of control, types of stimuli presented, and the long term study of single individuals through time. Such long term studies of animals, such as Jane Goodall's work on chimpanzees, and Cynthia Moss' work on elephants, have provided us with a 30 year run on the lives of highly social and fascinating creatures. It would be difficult to match such studies on humans.

So there are three ways in which studies of animals impact directly upon our understanding of the human mind, and these are now coming to play a much more dominant role in both the cognitive sciences and the neurosciences.

JB: What are you bringing to the table that's new?

HAUSER: The newness is both methodological and theoretical. The methodological is a little bit easier to identify. A large number of researchers working on animal cognition have had an almost myopic focus on making animals seem human. So we engage in all these exercises to show that through extensive training procedures we can make a pigeon look like a human. Now that is an interesting exercise, because it shows that at least at some level, the brain of an animal has the capacity to learn something, but what interests people working on humans, primarily about human thought, is what we do spontaneously in the absence of any training. It is clear that we can be trained to do extraordinary things. And that is an interesting aspect of the human mind. But what's more interesting, in part, is the spontaneous stuff because that's probably what's going to tell us most about the signature of evolution.

We are working with techniques that allow us to identify what animals spontaneously bring to the table in terms of how they think about the world. And that is a relatively new approach. It's not completely novel; people like Dorothy Cheney and Robert Seyfarth have been doing these kinds of experiments with animals in the wild, which clearly involved no training, and simply ask animals in their natural habitat what they are doing. We have taken advantage of those kinds of techniques and done similar experiments in captivity. The theoretical push that we have made is to unite evolutionary theory with modern ideas in cognitive science in a way which is very novel. One of the problems with evolutionary psychology is that it has had a very narrow focus on humans. Evolutionary psychology, broadly defined, has actually been going on since the days of Darwin. Darwin asked questions about the mind with an eye to evolutionary intuitions. What we're now seeing with the work that I've been doing is a real emergence of Darwin's initial intuition — that we can really marry, in a serious way, evolutionary theory with the cognitive sciences, as applied to the study of animal mind.

We ask questions about the design of the brain, the design of mental states by looking at how social behavior and ecology shape those processes. For example, we've recently been interested in the extent to which animals have a domain of knowledge that you might call naive physics. To what extent do animals make intuitive predictions about physical objects, based on the physics of the world. These kinds of questions would not have been asked if one were not sensitive to the kinds of statistical regularities that animals confront in their environment. It is true that all animals throughout our planet are confronted with the problem of gravity. When objects fall, they fall straight down — except of course if deviated by an object that gets in the way of the falling object. What we've been able to do is use the regularities that animals confront to ask questions about what kind of biases they bring to working their way through the world.

For example we have created an experimental procedure that follows along some of the work that's been done with human children, where you drop a ball through an opaque tube that has an S-like configuration. If you understand the way tubes work you should predict that the object will fall out at the end of the tube. Monkeys, and human children, expect the ball to land directly below the release point. They seem to be bringing gravity biases as a predictive force in their decisions. This is a problem, because it shows the great difficulty that children and some animals have inhibiting or suppressing a very strong bias that has been selected for because of the statistical regularities in the world. That is, gravity is a regularity that all animals on Earth confront. Consequently, rather than learn about the forces of gravity, I believe that selection favored brains that, innately, made predictions about falling objects. Because of this innate sense, it is difficult for animals to override their intuitions when the evidence goes against their beliefs.

This approach is new to the study of animal cognition, as well as the study of human cognition.What makes this approach powerful is that it then leads us to studies of the human brain. We therefore begin to create an intimate connection between the thoughts and the neural-mechanisms that underlie them. For example, why can't animals find the correct location of a falling object through a tube? Why can't they inhibit their biases, and search in a different location? We now know from studies of the evolution of the brain that the frontal parts of our brain have undergone extraordinary evolution over the past five to six or more millions of years. If we compare the relative size of the frontal regions of our brains, relative to a primate of our size, it's about two hundred percent bigger than you predict. So in a monkey species like the tamarins that I study in captivity, that part of the brain is greatly impoverished relative to its capacity in the human brain. What we find is that prefrontal region of our brain is the part of the brain that's engaged in blocking repetitive responses. So for example when unintentially bump into a glass door, we may do that once because we fail to notice that it's glass, but we won't repeat that error over and over again. We have a mechanism that's specifically designed to inhibit those kinds of actions. That is something that really did not evolve in a significant way within many non-human animal species. So what's exciting to me is that we really have a deep connection between the ecology and social behavior of the species, the mental states they bring to the task of solving the problems, and then a further step back, into the brain seeing how the brain evolved to meet those particular problems. And that's a unique marriage today.

JB: What other scientists do you relate to?

HAUSER: The people that I have the most convergence with, or interest in their thinking and work are probably the psychologist Stephen Pinker and the philosopher Dan Dennett. To finish the disciplinary areas, I would add Richard Dawkins. What has continued to amaze and intrigue me about these three is their incredible ability to explicate very complicated theories in evolutionary biology. When I was an undergraduate, my first course in evolutionary biology was based on Ed Wilson's recently published book Sociobiology. That was the first book I read in the field, and then immediately went on to read Dawkins's The Selfish Gene. What struck me was the richness of ideas in this field, and extraordinary clarity with which Dawkins expressed them. I was also impressed by the lack of empirical workat the time, but could immediately see how the empirical work would come. And it did, volumes and volumes of it. Dawkins has an exceptional ability to take these difficult ideas, which often required pages and pages of algebra to explain in the original papers, and provide just the most elegant metaphors to explain them. The downside of Dawkins' book is that people who don't understand the biology often read them and conclude that this is a very trivial, simple field, whereas they really haven't grasped the fundamental problems. I suppose this is why Richard, when asked why he keeps writing the same book, says that it's because people continue to fail to understand him. The fact that he writes so clearly makes it seem that the biology is very, very, easy, and that's a potential problem.

Steve Pinker is another one of these people who just have the most remarkable ability to think clearly about difficult problems and then write about them in an entertaining way. Steve also suffers from the same problems Dawkins suffers from. Critics often don't read in depth through the books or articles that Steve writes. They extract snippets or pull out columns Steve writes inThe New York Times, and then make faulty conclusions. Natalie Angier recently wrote a piece in The New York Times Magazine section, criticizing evolutionary psychology and pulling out Steve Pinker as one of the major problems for that field.

As an example, Natalie Angier's criticism of evolutionary psychology was based on a misunderstanding of the basic biology that leads to sexual promiscuity in humans — the biological fact that women and men are caught in a nasty asymmetry. At the get-go, sperm are cheap to produce and eggs are expensive. That means that women start out with an investment cost that males never pay which allows men the freedom to be promiscuous, and constrains women. That's not to say that women might not be promiscuous and men might not be monogamous, but you start out life with an assymetry. That's where many of the ideas in evolutionary psychologycome from, and that people like Steve Pinker discuss. Unfortunately, because the writing is so often loose and entertaining, it misses the key empirical facts. That said, what I appreciate about Steve's work is that he has brought to psychology the importance of thinking in an evolutionary way, following the lead of other researchers such as John Tooby and Leda Cosmides. Steve has also brought these ideas to a much broader audience. He is the most eloquent spokesperson for the cognitive sciences.

Last on the list, Dan Dennett I just thoroughly enjoy because of his creatively promiscuous mind, He thinks about deep problems in a clear way. And what I've always found enjoyable about Dan, maybe in part because we see each other often in a discussion group, is that he's willing to engage and entertain on any topic, and think about it in a hard way. Like Dawkins and Pinker, Dennett has brought to philosophy the importance of evolutionary thinking.

In sum, Dawkins, Dennett and Pinker make thinking fun. They show how good writing pays off in terms of educating. And they show how exciting science can be.

JB: Where does Stephen Jay Gould fit in?

HAUSER: He's somebody I enjoy reading, less in terms of impact upon me. Steve Gould and Dick Lewinton for years now have raised critical comments about what they call the adaptationist program. And they've done it in a way that is unfortunately really devastating for these different fields. Rose would be in the same category, but maybe less damaging than Lewinton and Gould because of the power that Lewinton and Gould have on the scientific community, as well as the public.

In the mid-1970s they wrote a famous papers on the spandrels of San Marco. This paper was a critique of socio-biology. What it effectively did was to pull out selective pieces of writing, often in popular books about socio-biology, and criticized the field as if everyone was a naive adaptationist. And they picked out people like David Barash and popular writings of E. O. Wilson, and criticized them for being naive with respect to their thinking about evolution, natural selection, and adaptation.

The problem it seems to me is that any smart person can go into any field and find idiotic papers. It would be easy for me to go into the field of paleontology, Steve Gould's area, or evolutionary genetics, Dick Lewontin's area, and find a really dumb paper and say, that's the field. And that's damaging, especially for a field that's beginning to brew. I realize that part of the reason for their strong response to socio-biology, and now evolutionary psychology, is that there is a potential undercurrent of what they perceive as biological determinism. And there is no question from my perspective, having taught this material to undergraduates at Harvard for several years now that there is a tendency for students to conclude, and therefore many people who read these books to conclude, that if you are arguing for a biological component to behavior that you are necessarily going to conclude that it is deterministic; that because you find an "is", you conclude there is an "ought". And that of course is a danger, and they're right to point that out. Unfortunately what happens is that they are picking on the popularizers of these areas, rather than going to the primary literature. For example, Lewinton has written two papers stating that we can't study how cognitive processes evolved. Unfortunately Lewinton is not sufficiently familiar with the actual work that's being done in these areas to make that claim. It's easy enough to pick on one paper by Steve Pinker and Paul Bloom and say, here's a paper on language evolution that hasn't made any contribution empirically to the field, and yet ignore the thousands of papers that have used evolutionary theory to fuel empirical work on the problem. So although I have nothing negative to say about their own empirical work, which I find wonderful, insightful, an intelligent, I do find it unfortunate that Gould and Lewontin have a mission, it seems, to absolutely blast out of orbit these growing fields — which is unfortunate given the power they hold with the lay public.

JB: So what's next for you?

HAUSER: What the next ten to fifteen years will bring for our work is first to show how interest in the human mind demands an interest in evolutionary theory; it will show that empirically in a way perhaps that no study of humans can actually really show. And the way it's going to show that, is by looking at the problem from a wide variety of perspectives, and from different levels of analysis. It's going to show how the theory of evolution leads to novel predictions about the mind, that could never have been derived from studies that don't use the insights from evolutionary theory. Second, it's going to show how we can truly marry studies of animal cognition with the neuro-sciences. Neuro-scientists to a large extent have a tendency to ignore the important variation between species. When they work on rhesus monkeys, they talk about "the monkey." There are several hundred species of primates, yet neuro-scientists refuse to acknowledge that. Our work will begin to overturn a common and dominant view in the neuro-sciences, which is to ignore species variation. We hope to convince the neuroscience community that variation is wonderful, it's the truffle of biology, Darwin's truffle. By looking at variation, we see the workings of natural selection in sculpting different kinds of minds.

JB: Who disagrees with you?

HAUSER: Who's attacking me? Who are my enemies? There are two camps of enemies out there, some explicit, some implicit. There are people working with animals who find that if you don't train and shape an animal to make it very very specific and robotic you can't possibly derive anything interesting about what they know. Therefore some of the new techniques that we're trying to bring to the table of animal cognition people find absolutely sloppy and uninsightful and not very informative in terms of what we know. There's an entire contingent of people who are trained to a large extent in the Skinnerian tradition, who find this new breed of approach to animal cognition very sloppy and uninsightful.

There are people who work with humans who are becoming converts but who find our work annoying because it forces them to rethink their claims about human uniqueness. They're not really enemies, they just our work frustrating.

The third category, which includes people like Frans De Waal and Daniel Povnelli who tend to work with chimpanzees, don't particularly like the fact that some of the work that we do on monkeys shows comparable abilities to their work on chimpanzees. In the same way that there is human chauvinism, there is chauvinism within the community of animal scientists that people working with chimps are doing much more important work than people working on monkeys. And this kind of hierarchical chauvinism continues all the way through the tree of life. If one is concerned with the design of the mind, and how different pressures lead to different kinds of minds, the variation between species is of utmost interest, and there is no species chauvinism. There is a common mission: to find out how evolution paved the way for different ways of thinking.

Reality Club Discussion

A quick follow-up to Liz Spelke's point. In hundreds of studies of children's Theory of Mind there is no evidence of sex differences or a female advantage. These include hundreds of studies of the very false belief task that Baron-Cohen, in excellent earlier work, showed was strikingly deficient in people with autism, as well as many other studies of other similar abilities. There are other individual differences, younger siblings, for example, show a consistent advantage over older ones. But not sex differences.

Curators’ Professor, Department of Psychological Sciences, University of Missouri-Columbia

Baron-Cohen's proposals regarding the potential origin of autism and the relation to sex differences in cognition are very interesting and provocative, and at the very least will spur the field to think about these issues in different ways.

I wonder though if 'systematizer' is really the best way to conceptualize the autistic mind in particular and the male mind in general. Systematizing and searching for within category laws suggests explicit, conscious mechanisms that can be focused on more narrow domains, such as knowledge of biology or engineering. There is no doubt that humans are good at this sort of cognitive activity, and there may be more males than females who obsessively engage in systematizing. But in general, there do not appear to be large sex differences in the ability to consciously represent information in working memory and to organize this information in meaningful ways (this is another ways of saying there aren't large differences in general fluid intelligence). Still, boys and men do focus on different aspects of their world than do girls and women and work to organize their worlds in different ways.

More precisely, there are sex differences in more fundamental and implicit cognitive systems that may better capture the male mind and perhaps the mind of many individuals with autism than systematizer. Most broadly, these implicit systems encompass the domains of folk psychology (e.g., theory of mind, face processing), folk biology (e.g., categorizing flora and fauna in the local ecology), and folk physics (e.g., navigation, cognitions but tool use). Each of these folk domains is composed of a number of more specialized systems that in total seem to capture the essence of the evolved human mind.

Girls and women typically outperform boys and men in folk psychological domains — they are better at reading facial expressions, gestures, and at language production, among other differences - and boys and men typically outperform girls and women in folk physical domains — they are better at most spatial tasks, navigating in novel environments, and have a better intuitive grasp of tools. The book is open regarding sex differences in folk biological domains, although I suspect a male advantage in some subareas and a female advantage in others.

In any case, some of the patterns described by Baron-Cohen here and elsewhere (e.g., the results for visualization, over-representation of the relatives of autistic individuals in engineering) suggest that the autistic mind — and that of males — may be better described as being biased toward folk physics, at a cost to folk psychological systems.

Finally, I'd like to point out that many sex differences in general can be conceptualized in terms of Darwin's sexual selection, that is, competition with members of one's own sex for access to desired mates, and mate choice. The advantage of girls and women in many folk psychological domains can be understood in terms of female-female competition over boyfriends, would-be husbands, and other resources. Girls and women compete by gathering as much information on other people as they can get and then using this information to attempt to organize their web of social relationships so as to have better control of these relationships and through this access to what they want. This is called relational aggression. This way of 'fighting' will elaborate folk psychological brain and cognitive systems and eventually result in a sex difference in the same way that male-male physical contests over mates will result in the evolution of bigger and stronger males. Male-male competition may have favored an elaboration of folk physical domains in boys and men, because these support common survival — and reproduction-related activities of men in traditional societies and presumably during human evolution (e.g., navigation in novel terrain to hunt or engage in raiding parties). These types of evolutionary mechanisms provide a very useful big picture framework for hormonal studies that focus on proximate mechanisms.

Author, Machines Who Think, The Universal Machine, Bounded Rationality, This Could Be Important; Co-author (with Edward Feigenbaum), The Fifth Generation

Marc Hauser's interview was both a delight and provocative. His work with animals makes fresh approaches to some of the most vexing questions facing the field of human cognition and for that, it was a joy to read.

However, I'm puzzled by what he calls Natalie Angier's misunderstanding of evolutionary psychology based on the basic biology that leads to sexual promiscuity in humans, what Hauser calls "a nasty asymmetry" — that sperm are cheap to produce and eggs are expensive, and that therefore men have the freedom to be promiscuous (it's cheap), and women do not (it's costly).

Does Hauser — do evolutionary biologists or psychologists — impute to humans some instinct that has told us about this cheap/costly ratio (real evidence of which must have come very recently indeed in our evolution)? Is it that hence we have selected for females "naturally" choosing monogamy, and males "naturally" choosing promiscuity?

It's possible, I suppose — we've certainly heard the argument ad infinitum from guys who ought to know — but I take Angier's point to be that other interpretations of our mating patterns are at least as plausible. These alternative interpretations have the advantage of not confusing libido with procreation (connected of course, but by no means the same thing); nor confusing science with the social convenience of the long-dominant sex. In short, right or wrong, Angier brings to that particular issue the same kind of fresh and persuasive thinking that Hauser brings to cognition, and I'm surprised he doesn't see that.

It is hard to imagine anyone living today disagreeing with Baron-Cohen's starting premise that there are biological differences between the sexes. Even the staunchest cultural relativists have to acknowledge that there are differences in the sex chromosomes and hormonal titers that lead directly to differences in our anatomy.

Recent work on imprinted genes a class that fails to follow the classic Mendelian patterns of inheritance shows that maternal contributions are often in complete conflict with paternal contributions. For example, with some imprinted genes, the maternal copy is quiet and the paternal copy is expressed, causing the fetus to extract more from its mother than she would like; these genes often cause pregnancy complications such as gestational diabetes. Studies of the brain using neuroimaging reveal sex differences in structure and function, and work with patient populations reveal differences in vulnerability to mental disorder. And closer to home, there are massive sex differences in the incidence of autism, with studies reporting an 8:1 bias in favor of males.

Where the debate gets interesting is when one attempts to explain how tightly the biology constrains our thoughts, preferences and actions. Baron-Cohen's assortative mating hypothesis is an attempt to grapple with this issue. Much of the evidence hinges on the early appearance of sex-specific signatures of mental function. Early signatures are a tell-tale sign of an innate capacity peaking through, but they are not definitive. One needs to rule out that the experience obtained is insufficient for a learning mechanism to create the capacity.

And here is where Baron-Cohen's observation that newborn boys like to look at mobiles and little girls at faces is fantastic, and just the right kind of start into a serious research program on the biology of sex differences; these results fit nicely with other data showing that for spontaneously generated paintings by young children, little girls almost always draw one or more people into the scene, whereas little boys rarely do, using their canvas as a vehicle for vehicles, from rocket ships to more mundane cars and bicycles.

But now comes the hard work.

What is it about the male genome that sets up a preference for the mechanical or physical whereas the girl genome leans toward faces and the social? How quickly, and with what kind of experience, can these initial biases be exaggerated? Why did these differences evolve? In the language of Darwin, what selected for this kind of preference? Was it our division of labor, with males focused on hunting and therefore technology, while females focused on gathering and the schmoozing that goes on during this kind of activity?

One clue that these are evolved sex differences comes from recent work looking at the incidence of innovation among primates. Across all the primates, including our closest relatives the chimpanzees, males are far more likely than females to take the lead in innovation, and much of the creativity lands in the domain of tool technology. In contrast, for most primate societies, females are for more engaged in the intricacies of social life than are males, largely because females tend to stay in their natal groups for life whereas males emigrate out. If there is a bias toward male folk physics and female folk psychology, there may be traces way back to our primate ancestors.

How are data like Baron-Cohen's reconciled with the fact that for imprinted genes, maternally active copies appear to be largely expressed in the rational frontal lobes whereas the paternally active copies appear to be largely expressed in the emotional limbic lobes? Are there in utero battles that arise over the concentration of testosterone circulating during development, with paternally active genes pushing hard for increased testosterone to push growth and toughness? Are maternal copies pushing in a different direction, attempting to regulate the physiology in such a way that their offspring are social specialists?

What makes work like this so very difficult, especially in terms of selling it to the public, is that more often than we would like to admit, reported sex differences either crumble in the face of follow up work, or for those differences that have been reported and replicated, claims regarding biological underpinnings have fallen prey to more experientially-based accounts. One only need think back to gay genes and gay brains, and the sad fate of those results. Thus, although I am sympathetic to Baron-Cohen's research project and find it odd that anyone would consider this work controversial, there is an obligation to get the story right here that far exceeds the demands in other areas.

I find Simon Baron-Cohen's work admirable in several ways. The systematizer-empathizer dimension is an interesting new way to capture some of the variation between male-typical and female-typical cognitive styles. It cuts across motivation and aptitude (which are often difficult to distinguish) and might subsume some of the long-noted sex differences that have been stated in cruder form, such as orientation to objects versus people. I also am intrigued by the studies of the effects of fetal testosterone, a valuable new way to learn about which psychological sex differences might be consequences of the biological programs that build our brains.

Baron-Cohen wonders why sex is so often referred to these days as "gender." Part of it is a new prissiness — many people today are as squeamish about sexual dimorphism as the Victorians were about sex. But part of it is a limitation of the English language. The word "sex" refers ambiguously to copulation and to sexual dimorphism, and it's often important not to confuse them! The linguistic term "gender" literally means "kind," as in the cognates "genus," "generic," and "genre." Languages often subdivide their nouns into kinds for purposes of inflection, such as human/nonhuman, animate/inanimate, long/flat/round, vowel-final/consonant-final, and male/female. Many Indo-European languages have a gender distinction in their nouns that aligns with a masculine/feminine distinction in their pronouns, and so "gender" was pressed into service as a way to refer to the difference between men and women. Some academics want "gender" to refer specifically to socially defined rather than biologically determined patterns of sex-typical behavior, but this guideline, like most top-down prescriptions about lexical semantics, is rarely obeyed. The basic problem is that we have three concepts to convey — intercourse, dimorphism, and social roles — and at best two words with which to convey them.

I was amused to read that "It may be simply that the climate has now changed, and that people are much willing to accept that there are sex differences in the mind, and that these might even be partly biological." Was this interview conducted before the event that is coming to be known as "1/14"?

I take the premise of Simon Baron-Cohen's project — that there are innate sexual differences in behaviour and aptitudes — for granted. As Olivia Judson recently pointed out in a New York Times piece about "1/14", it "s hard for a zoologist to suppose otherwise.

I am not, however, wholly convinced by his argument that autistic children — nearly always boys — are, in effect, hypermales. Baron-Cohen has shown that, relative to girls, boys are good at systematising and poor at empathising, and that autistic boys are exceptionally so. This fascinating result then raises a question, namely, why should these two, seemingly unrelated, attributes should trade-off with each other?

Baron-Cohen's answer seems to be: foetal testosterone. Perhaps autistic boys were exposed to unusually high levels of testosterone in the womb, so developing the systematising part of their brains, and repressing the empathising part. It's an exaggeration of a normal process. This strikes me as perfectly plausible, but it also entails a number of peculiar, if testable, consequences.

Testosterone is a hormone and, as such, affects the entire brain and body of the foetus. This means that one should expect autistic boys to be hypermales not just with respect to systematising and empathising — but in all possible ways.

Are autistic boys, then, exceptionally aggressive at play? When they grow up, are they invariably heterosexual? Do they look exceptionally masculine — that is, are they morphometrically hypermale? And what of those rare autistic girls? If they, too, are hypermale due to heavy doses of foetal testosterone, should this not be reflected in their behaviours, their sexual orientations, their bodies?

The motivation for these questions is that we do know something about the consequences of high foetal testosterone — witness the Spotted hyena. In all mammals, the placenta produces a lot of testosterone. This testosterone is, however, broken down into oestrogen by an enzyme called Aromatase. Spotted hyenas foetuses have naturally low levels of Aromatase, and so are exposed to extraordinarily high levels of placental testosterone. The result of this is that females are born with large penis-like clitorises that they can jaunt erect in dominance displays, fused vaginas and very nasty tempers.

Now, clearly we can't ask Spotted hyenas to systematise and empathise. But we can ask their human equivalents. Loss-of-function Aromatase mutations are occasionally found in humans. Like the hyenas, girls homozygous for such mutations are born with masculinised genitalia. Nothing, I think, is known about their temperaments or talents. Are they autistic? Better yet, are their brothers homozygous for the same mutations, autistic as well? If Baron-Cohen is right, they should be.

Of course, even if the hypermale theory of autism is wrong, Baron-Cohen's proposed mechanism of assortative mating among systematisers could still be right. But let us hope not. Surveying my colleagues it seems to me that the reproductive success of gifted female scientists is poor enough as it is.

Simon Baron-Cohen's Assortative Mating Theory of Autism is ambitious, and demonstrates the usefulness of innovative, "big picture" thinking in getting a handle on seemingly intractable problems. The ideas seem to fit together nicely to present a clear picture of the etiology of autism. The flip-side of big-picture thinking, however, is that the authenticity of the full picture relies on the validity of each sub-theory; and in this case, the big-picture is painted with a series of appealing, yet tentative strokes. Baron-Cohen acknowledges that the underlying theories need to be tested, and he presents five sub-theories, in the form of testable hypotheses, which focus primarily on the relationship between genetics and systemizing.

His main idea is as follows: people process the world using a cognitive style that falls somewhere along the spectrum of systemizing (more masculine) to empathizing (more feminine). In systemizing, information is processed with attention to rules and laws, and in empathizing, processing is biased toward social cues. As autism can be characterized by the combination of two extremes — a lack of attention to social cues and a focus on laws and rules — then it follows that, should there be genes for systematizing, two systemizers have a higher chance of producing an autistic child. It further follows that, should systemizing be a predominantly masculine trait, the agent that masculinizes the brain, prenatal testosterone, may be also play a central role in both systemizing and autism.

A wealth of evidence, including Baron-Cohen's own research, links the effects of early testosterone to later masculine behavior in humans and non-human animals. Some of the most robust research has examined the effects of Congenital Adrenal Hyperplasia (CAH), in which too much testosterone is produced by the adrenal gland prenatally. Females with this condition (whose hormonal levels are normalized at birth) are somewhat masculinized—e.g., they engage in relatively high levels of rough and tumble play and perform in the male-typical range on spatial tests (higher than normal females). If early testosterone increases systematic processing and masculine behavior, then it follows that CAH boys should also show increased performance on tests of spatial ability; but among males, the relationship between perinatal testosterone and later spatial performance is equivocal at best. Indeed, there is evidence to suggest that excess testosterone in utero actually hampers male spatial ability.

A big-picture, evolutionary analysis of the Assortative Mating theory reveals somewhat of a paradox between conventional notions of masculinity, and the newer notions of "cognitive masculinity." Testosterone can be thought of as promoting behaviors that are traditionally masculine, preparing males physically and psychologically to bias energetic investment toward mating effort. In adult males, high testosterone levels are associated with status-seeking behaviors and the pursuit of mating opportunities. In men, confidence and social dominance (which would require a relatively high social facility) are predicted by high testosterone. The case of the classic nerdy scientist conjures up images of the stereotype of the low testosterone, but in the current context "cognitively masculine," man — a scrawny male who, although he may be successful in the world of technology, is a miserable failure socially and romantically.

The paradox of the two notions of masculinity raises questions about the role of testosterone in shaping psychological traits, such as status-seeking behavior and spatial ability, in utero and in adulthood. With my colleagues Chris Chabris, Peter Ellison and Steve Kosslyn, I've investigated the role of testosterone in solving spatial problems. We have found that although high testosterone males outperformed low-testosterone males on mental rotation tests, the high performers gained their advantage not because they were better at internally transforming objects, but, as the evidence suggested, because they were more confident in their decisions about the similarity of objects. Perhaps the paradox can be at least partially resolved by furthering our understanding of testosterone's role in affecting performance on cognitive tests.

These findings on individual differences in mental rotation performance, along with a relative lack of robust findings on the effects of perinatal testosterone, remind us that picture of how testosterone affects cognition is still far from complete. While evidence strongly suggests that early and late sex differences in testosterone levels play a central role in shaping traits that are clearly related to mating effort, such as dominance, sexual, and competitive behaviors, we should not take for granted that testosterone affects cognitive ability directly. More research should examine the extent to which the relationship between testosterone and cognitive performance is actually modulated by a third variable, having to do with dispositional factors such as confidence and competitiveness, that are more closely associated with mating effort.

Professor of Psychology and Co-Director of the Mind, Brain, and Behavior Initiative

We humans seem to have an abiding need to reduce the richness, variety, and complexity of our mental lives to categories. In past times, we divided ourselves into the introverts and the extroverts, the field-dependent and the field-independent, the visualizers and the verbalizers. With the advent of neuroscience came the chance to divide our brains into categories as well. That supremely intricate and elusive organ became the left brain and the right brain, the grey matter and the white matter, the male brain and the female brain. Simon Baron-Cohen builds on the last distinction and offers us a new pair of categories by which to sort ourselves: we are systematizers or empathizers.

The categories of the past have a quaint look about them, because none of them has proved very satisfying. Binary categories don't buy us much, because humans are both more and less variable than they suggest. Two categories are far too few to pigeonhole a species whose members can grow from newborn infants to seal-hunters, cathedral builders, or capoeira dancers. Yet two categories are also one too many. In the right circumstances, all of us become introverted or extroverted, swayed by others or steadfast to our principles, visually imaginative or verbose. Global categories tend to obscure the commonalities in human experience and the common capacities, hopes, and failings that define us as a species.

Is there a male brain, and is systematizing its job description? Baron-Cohen asks us to distinguish politics from science and consider the evidence. The evidence for an inborn, male predisposition for systematizing comes from a single experiment on newborn infants, tested with a single person and object. The person was the report's first author, who surely knew the experimental hypotheses and who, we now learn, may have known the sex of the infants whose attention she elicited. The experiment provides no evidence that the basis of infants' preference, if real, had anything to do with the categorical distinction between the displays. Would infants show the same preferences for other face/object pairs? Would they maintain this preference if low-level properties of the two displays, such as their speed of motion, were equated? One need not object to Baron-Cohen's politics to be less than persuaded by his data.

More important, Baron-Cohen fails to consider the extensive evidence that has accumulated, over the last thirty years, on infants' developing understanding of object mechanics. Hundreds of well-controlled experiments reveal no male advantage for perceiving objects or learning about mechanical systems. In most studies, male and female infants are found to discover the same things at the same times. Both males and females come to see the complete shapes of partly hidden objects under the same conditions and at the same ages. They figure out how objects support one another, through the same series of steps. They reach for objects by extrapolating their motions, with equal accuracy. They make the same errors when they search for hidden objects, and they get over those errors at the same time. Sometimes female infants have an edge: In experiments by Laura Kotovsky and Renee Baillargeon, for example, females start to learn about the relation between force and acceleration (the harder a stationary object is hit, the further it goes) a month earlier than males do. Males catch up, however: by 6 1/2 months, you can't tell them apart.

Whatever the newborn infants in Baron-Cohen's experiment were doing, the male and female participants in three decades of infant research have followed a common path, engaging with objects and people. Infants don't choose whether to systematize or empathize; they do both, and so do we all. Baron-Cohen's categories may seem as quaint as left and right brains by the time his newborn subjects are old enough to read about them.